The well may be free flowing or may be started by means of lift gas injected to a down-hole location to reduce the density of the well effluents in the production tube. Dynamic control of lift gas injection into a production tubing is known from European patents No. 840836 and 945589 and from UK patent applications No. 2342109 and 2252797. These prior art references do not disclose measures to deal with effects of gas coning.
It is known that gas coning occurs if oil is produced from an oil bearing formation layer that is relatively thin and located below a gas bearing formation layer which is often referred to as the gas cap. A lower pressure at the bottom of the vertical production tubing will draw progressively harder on the oil in the reservoir, eventually drawing gas from the gas cap above the oil bearing formation layer.
It is common practice to adjust the size of the surface choke or bean such that the phenomena of gas coning is mitigated, thereby preventing the depletion of the gas cap and avoiding that the production of crude oil is reduced and the well predominantly produces gas from the gas cap.
If the critical drawdown is still exceeded unintentionally, possibly as a result of gradual changes to the reservoir, full gas coning will develop. This leads to substantially lower oil production and unwanted depletion of the gas cap. When gas coning has developed fully, it can only be stopped by taking back the well production substantially, leading to deferment.
International patent application WO98/25005 discloses a method of producing crude oil wherein a small amount of gas from a gas cap is injected into the production tubing and the gas injection rate is controlled by a downhole one way check valve, which is not adjustable and wear prone.